Carbon dioxide purification process



am@ l WW April 10, 1951 W, H, Rupp 2,548,499

CARBON DIOXIDE' PURIFICATION PROCESS Filed Feb. 17, 1948 IMDUQE C@2 @As INLET i SUTLUEQ UNcoNDr-:NSEV @A H fw l Q. i COOLEQ-J O [A72 CO2 l i L MM @1b fz/ 'Mm' :52

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Patented pr. l0, 195i 2,548,499 CARBON DIOXIDE PURIFICATION PROCESS Walter H. Rupp, Mountainside, N. J., ass'ignor to Standard Oil Development Company, a corporation of Delaware Application February 17, 1948, Serial No. 8,844

. 8 Claims. l

This invention relates to a purication process for carbon dioxide, and is particularly vapplicable to the purication of carbon dioxide to `be used in the manufacture of Dry Ice. In particular, this invention concerns a liquid phase CO2 purication process in which impure carbon dioxide is contacted with the C2 hydrocarbons, ethane and ethylene, either separately or in admixture.

Carbon dioxide is obtainable from several sources. For example, it may be obtained from certain carbon dioxide rich natural gases, or as a by-product of a fermentation process, or by the. combustion of various gases, cokes or oil fuels, or by the decomposition of limestone. The principal use of CO2 is in-the manufacture of the solid refrigerant known as dry ice. For this use it is essential that the nal product be oi an extremely high purity for the reason that Dry Ice often comes in direct contact with foodstuis. Consequently, it is generally essential to highly purify the carbon dioxide irrespective of the particularsource of the CO2.

At the present time a good many complicated methods of purification are used in the Dry Ice industry. characterized by the fact that the purification of the CO2 is carried out in gas phase. In general three steps are required to attain the necessary purication. The rst step .may comprise the removal of sulfur compounds generally present in the impure carbon dioxide obtained from the sources mentioned. The sulfur compounds may be removed by a variety of chemical reagents. For example, lead or Zinc acetate solutions, sodium carbonate, sodium bicarbonate, caustic, diethanol amine, or other reagents maybe employed. A second purification step is theremoval of heavy oils and organic liquids andparticularly of aromatic oils. Very small traces 'of heavy oils will impart unfavorable taste and odor toCOz, or to the Dry Ice formed from CO2. In order to effectively eliminate the heavy oils, therefore, gaseous CO2 is conventionally contacted with charcoal, silica gel, etc. or alternatively the CO2 is carefully fractionated from the heavy oil. The third treating step conventionally required is the removal of excess water. It should be noted that it is not desirable to remove all water, since a small proportion is desirable in order to impart good crystalline structure to the Dry Ice formed. However, excess water is objectionable in tending to freeze control valves, heat exchangers, etc. Generally solid drying agents are employed to remove undesired vWater fromimpure CO2. Alu- All of these methods, however, are

2 Y mina, fiorite, sulfuric acid, silica gel, and other agents may be used. f

IIt will Vbe noted that in the conventional purification process outlined above, three purication steps are required. As stated each of .the steps is carried out using gaseous carbon dioxide. Itis a particular object of this invention to enable suitable -'puriiication Ato be lmade in a one step process and further tocarry out vthe purication with the carbon dioxide in the liquid phase rather than the gaseous phase. The lattercharacteristic 1of this invention is Ydesirable in permitting iinal Dry Ice productionemploying one compression step, thus eliminating any necessity for. recompression during the process. Other objects and advantages of this-invention will become apparent from the following detailed description in connection with the accompanying drawing in which is illustrated theflowplan of thepurication process of this invention.

Referring to thedrawing, CO2 gas containing undesirable impurities as derived from any source such as the fermentation of sugar or combustion of coke is introduced through line l, to a suitable cooler 2. This CO2 gas must be under sufficient pressure to permit condensation at suitabletemperatures. If the source of CO2 does-not provide the CO2 gas at asufiiciently high pressure, a-conventional type compressor, not shown inv `the drawing, may be employed. As illustrated the cooler 2 may comprise a simple heat exchanger employing cold water or an equivalentcooling medium to cool the CO2-containing gas suiiicently to permit condensation under the pressure existing in the system. The cooler may, forexample, be effective in dropping the temperature of the carbon dioxide to a range of 32 to 80 F. which will cause condensation at 500 to 1100 p. s. i. g. pressure respectively. If the water content of the carbon dioxide gas is substantially zero, the temperature can be lowered to 0 F. at 300 p. s. i. g. pressure to cause condensation. The cooled carbon dioxide is then conducted through line 3 to separator l, wherein the vcarbon dioxide is collected as liquid CO2. It 'may be noted that the presence of' lower boiling gases than carbon dioxide tends to lower the condensation temperature required at a xed pressure. During condensation, water or ice and some hydrocarbons tend to separate and may be withdrawn from the separator through line 5. Also xed gases, or gases that are not condensed under v thc conditions of temperature and pressure employed may be withdrawn from the separator through line. `:The liquid carbon dioxide vis-then freed of all contaminants to be removed from the carbon dioxide. Either ethane or ethylene may be employed, or mixtures of ethane and ethylene. In either case the liquid carbon dioxide ows downwardly through the tower contacting upiiowing C2 hydrocarbons at temperatures ranging from 0 to 80 F. and pressures of 3D0-1100 p. s. i. g. A preferred range is 32 to 80 F. and 500 to 1100 p. s. i. g. The contacter or extraction tower may comprise a packed tower, or may consist of a perforated plate tower, or may comprise a high energy input mixing device such as a turbo mixer The only requirement here is that tower 8, or its equivalent, be eiiective in securing intimate contact of the liquid C2 hydrocarbons with the liquid CO2. The liquid C2 hydrocarbons are effective in extracting sulfur compounds, heavy oils, and water from impure CO2. Thus the C2 hydrocarbons removed from the top of tower 8 will comprise impure C2 hydrocarbons contaminated with sulfur compounds, heavy oils and water initially present in the impure carbon dioxide. These materials withdrawn from line II, are introduced to settler I2 wherein principally CO2 is permitted to settle out and is returned to the extraction tower through line I3. The impure C2 hydrocarbons are removed from the upper portion of the settler and may be withdrawn through valve I4 and line I5, and may be used as fuel gas, feed stock to another process etc. or may be withdrawn through valve I6 and line I I for re-purication and recycling to the extraction tower. 'I'his re-puriiication of the C2 hydrocarbons will be hereinafter described. Puried carbon dioxide, containing some C2 hydrocarbons, is withdrawn from the bottom of the extraction tower 8 through line I8 and is introduced to settler I9. The upper phase separating in this settler comprises essentially C2 hydrocarbons which may be withdrawn through line 2Q and may be returned to the extraction tower. The lower phase in the settler is withdrawn through line 2| and is passed through a throttling valve 22 wherein the pressure of the liquid to solid carbon dioxide and shaped into blocks known to commerce as dry ice.

As stated, the impure C2 hydrocarbons removed from extraction tower 8, may be suitably puried so that the hydrocarbons may be recycled in the purification system if desired for economy reasons. For this purpose the impure C2 hydrocarbons are conducted through line Il to a drying tower 24. This tower can be a chamber containing alumina or some drying agent or alternatively can be a conventional fractionation tower employing conventional bottom reboiling and top refluxing. The tower should have about 20-40 plates and can operate from 300 to 600 p. s. i. g. pressure. In this case, fixed gases dissolved in the C2 hydrocarbons such as methane together with carbon dioxide and water vapor will be substantially removed overhead through line 25 at a temperature of 0 to 70 F. while C2 hydrocarbons and heavier hydrocarbons are removed from the bottom of tower 24 through line 26 at a temperature of 15 to 85 F. The bottoms of tower 24 are then conducted through line 26 to conventional fractionator 21, which is of similar construction to tower 24 and operates at 28o-580 p. s. i. g., preferably 500-580 p. s. i. g. pressure. The fractionation carried out in tower 21 is effective to cause Ca hydrocarbons and higher hydrocarbons together with at least a portion of the sulphur contaminants, shown on the drawing as heavy oils, to be produced as a bottoms product so as to be withdrawn through line 28 at a temperature of 100 to 200 F. The substantially pure C2 hydrocarbons are then removed overhead through line 28 at l5 to 85 F. temperature. Depending upon the sulfur content of the original carbon dioxide puriiied it may be necessary to contact the C2 hydrocarbons of line 29 with a suitable chemical reagent to insure low sulfur content hydrocarbons. As illustrated, if this is desirable, or necessary, the C2 hydrocarbons of line 29 may be passed through valve 30 and line 3i to the sulfur removal tank 32 wherein the hydrocarbons are contacted with a suitable chemical reagent to extract the sulfur compounds. This agent may consist, for example, of caustic, sodium carbonate, or diethanol amine. The C2 hydrocarbons purified as described are then removed from sulfur removal vessel 32 through line 33, or are caused to bypass vessel 32 through line 34 and are returned to the extraction tower 8, through lines I0 and 35.

In general about 10-20072, of liquid C2 hydrocarbons, by volume based on liquid carbon dioxide, is necessary to suitably purify the carbon dioxide. The exact percentage required depends entirely upon the Volume of contaminants present. For the purification of carbon dioxide which is obtained in relatively pure form, this process will be satisfactory for purifying the carbon dioxide in a once through ethane or ethylene treating process using only about 1050% C2 hydrocarbons by volume on liquid CO2. This is particularly true of carbon dioxide derived from natural wells. In the United States and various parts of the world, there are a number of prolic wells producing natural gases containing up to 97% carbon dioxide. Typical compositions of such gases are indicated in Table I.

TABLE I Mol percent Methane 1 5 Ethane l 5 Carbon dioxide 97.7- 89.3 Propane and heavier 0.2- 0.5 Sulfur compounds (H25, mercaptans) 0.1- 0.2

75 of natural gases indicated, purification according to this invention may be readily carried out using a C2 hydrocarbon treat of about 10-50%.

As described, this invention consists of a liquid carbon dioxide purification process wherein liquid carbon dioxide is contacted with liquid C2 hydrocarbons. Intimate contact of these liquids is eiective to substantially purify the carbon dioxide, eliminating sulfur compounds, excess water, and heavy oils. This purication process is applicable for the purication of carbon dioxide as obtained from any source, and for whatever purpose the carbon dioxide is to be used. As brought out, however, the purication process is particularly adapted for use in the manufacture of Dry Ice, and is furthermore, particularly effective in the purification of natural gases rich in carbon dioxide.

While the process has been described as being suitable for securing substantially pure carbon dioxide suitable for conversion to Dry Ice, it must be understood that the teachings of this invention may be broadly applied to purify carbon dioxide to a lesser extent. For example, the carbon dioxide may be purified with less than the preferred quantities of liquid C2 hydrocarbons so as to facilitate throughput, and to secure a somewhat less puried carbon dioxide. Again, while it is preferred that liquid C2 hydrocarbons be employed in the contacting step, it is within the teachings of this invention to employ C3 and even C4 hydrocarbons for this purpose. The limitation on employing C3 and C4 hydrocarbons lies in the difficulty of freeing these hydrocarbons from the liquid carbon dioxide after the contacting step. Consequently, when employing these hydrocarbons rather than C2 hydrocarbons, the separation attained in the settler I9 will not be as great, and the separation obtained on flashing the carbon dioxide with residual traces of the hydrocarbons will likewise not be as great. How# ever, it is intended that the process described may be employed with C3 and C4 hydrocarbons as well as with C2 hydrocarbons. The carbon dioxide gas obtained in this case, however, would not be sufciently pure to provide the best quality Dry Ice.

Having now fully described the invention, what is claimed is:

1. A process for the purification of carbon dioxide in which impure liquid carbon dioxide is intimately contacted vwith liqueed hydrocarbons having two carbon atoms, whereby the said impure carbon dioxide is substantially purified of sulfur compounds, water and heavy oils.

2. In the manufacture of Dry Ice wherein natural well gases, rich in carbon dioxide are employed, the improvement which comprises purifying said natural gases by liquefying said gases and thereafter contacting the said liqueed gases wtith liquefied hydrocarbons having two carbon a oms,

3. The process for purifying carbon dioxide which consists of the steps of contacting said carbon dioxide in liquid phase with liqueed hydrocarbons having two carbon atoms, withdrawing the said hydrocarbons employed in said contacting step and fractionating them 'whereby substantially pure hydrocarbons are obtained, and recycling said substantially pure hydrocarbons to the contacting step.

4. The process defined by claim 1 wherein about 10-200% of the said hydrocarbons are employed per volume of liquid CO2.

5. The process defined by claim 1 wherein the said hydrocarbons consist principally of ethane.

6. The process for the purification of carbon dioxide contaminated with sulfur compounds, water, and heavy oils in which the said impure carbon dioxide is contacted in the liquid phase with liquefied hydrocarbons having two carbon atoms at a temperature of 0 to 80 F.

7. The process for the purication of carbon dioxide contaminated with sulfur compounds, water, and heavy oils in which the said impure carbon dioxide is contacted in the liquid phase with liquefied hydrocarbons having two carbon atoms at a pressure of 300 to 1100 p. s. i. g. and at a temperature of 0 to 80 F.

8. A process for the purication of carbon dioxide in which impure liqueed carbon dioxide is intimately contacted lwith liquefied hydrocarbons selected from the group consisting of hydrocarbons having 2 to 4 carbon atoms inclusive, whereby the said impure carbon dioxide is substantially puried of sulfur compounds, water and heavy oils.

WALTER H. RUPP.

REFERENCES CITED K The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,730,805 Claude Oct. 8, 1929 1,804,432 Pollitzer May 12, 1931 2,020,189 Jones Nov. 5, 1935 2,039,330 McKee May 5, 1936 2,088,497 Tijmstra July 27, 1937 2,327,134 Schuftan Aug. 17, 1943 2,378,762 Frey June 19, 1945 OTHER REFERENCES Faradays Encyclopedia of Hydrocarbon Compounds Ci to C5, under C21-Is, Sheet Number 0201900.96.

An Outline of Organic Chemistry, 1937 ed.. pp. 28, 32, by Degering, Nelson and Harrod. Barnes & Noble, Inc., N. Y. 

8. A PROCESS FOR THE PURIFICATION OF CARBON DIOXIDE IN WHICH IMPURE LIQUEFIED CARBON DIOXIDE IS INTIMATELY CONTACTED WITH LIQUEFIED HYDROCARBONS SELECTED FROM THE GROUP CONSISTING OF HYDROCARBONS HAVING 2 TO 4 CARBON ATOMS INCLUSIVE, WHEREBY THE SAID IMPURE CARBON DIOXIDE IS SUBSTANTIALLY PURIFIED OF SULFUR COMPOUNDS, WATER AND HEAVY OILS. 